A genetically obsese strain of mice developed at Colorado State University during more than 40 generations of selection for increased appetite will be compared to a control strain, derived from an identical genetic base, on three diets--high carbohydrate, low fat; high fat, saturated (beef tallow); high fat polyunsaturated (corn oil). The pattern of development of the adipose cells will be monitored from early in life through adulthood and old age on each diet, to determine whether adipose cell number is in fact fixed genetically at any point in the animal's life, and whether it is modifiable by diet. Monitoring will be done by direct observation under a light microscope, so that as far as possible small or unfilled cells do not go undetected. Effects of starvation and refeeding on cell size and number will be tested. Effect of composition of diet on food intake and its consequent role in the development of obesity will be observed. Effects of strain and diet on longevity, body composition over age, fatty acid composition, and fatty acid synthesis will be determined.